The piezoelectric ceramic speaker works in a principle distinct from that of the electro dynamic speaker. In the piezoelectric ceramic speaker, varying voltage is applied to the piezoelectric material to result in deformation of the piezoelectric material and vibration of the air to generate sounds. The electro dynamic speaker converts electric energy into magnetic energy and further converts magnetic energy into mechanical energy. However, the piezoelectric ceramic speaker directly converts electric energy into mechanical energy so that it has higher energy conversion efficiency. The electro dynamic speaker uses coils, generates electromagnetic field, consumes a lot of power, and generates electromagnetic interference. Contrarily, the piezoelectric ceramic speaker is free of all the above-mentioned problems and has the advantages of compactness and low power consumption. Therefore, piezoelectric ceramic speakers are widely used in smart phones and small electronic products.
The piezoelectric ceramic speaker generates high-frequency sounds but not low-frequency sounds. An approach to increase waterproofness of the piezoelectric ceramic speaker is to increase the density of the ceramic plate therein. However, increasing the density of the ceramic plate also increases the frequency of the sound generated by the piezoelectric ceramic speaker, which is shrill and harsh to the ears. Taiwan patents of No. M415529, No. M426235, and No. I274522 disclosed piezoelectric ceramic speakers. Each of them has a positive contact and a negative contact which respectively connect with a positive loop and a negative loop. The above-mentioned loops have at least two power wires not wrapped by plastic. Water may induce electric conduction between the exposed power wires and result in short circuit and damage of the piezoelectric ceramic speaker.
In order to secure the ceramic plate to a firm baseplate but still allow the ceramic plate to vibrate after receiving signals, a vibratile medium is interposed between the ceramic plate and the baseplate, and a space for vibration of the ceramic plate is formed in the baseplate. Thereby, the ceramic plate resonates with the vibratile medium in the space to generate sounds. For an example of the vibratile medium, it can be an elastic pad made of rubber or foamed plastic and connected with the bottom of the ceramic speaker in the above-mentioned Taiwan patent No. M426235. However, the conventional piezoelectric ceramic speaker has only a single vibratile medium, which is less likely to achieve a satisfactory resonance effect.